This invention relates to apparatus for assisting hearing in humans and animals, a bone implant and a hearing aid device.
A significant proportion of the population suffer some form of hearing impairment. This can be due to a variety of factors, including as a result of disease, genetic conditions, congenital malformations, removal of an acoustic neuroma and trauma. Hearing impairment can be due to problems in one or more of the outer, middle and inner ears which together form the human auditory system. Hearing impairment may be bilateral (on both sides of the head) or unilateral (occurring on only one side of the head).
Various devices have been developed to address hearing impairment. For patients with mild hearing loss, commonplace air conduction hearing aids are often sufficient. Air conduction hearing aids provide amplified sound into the ear canal so as to compensate for reduced hearing sensitivity. Sound pressure delivered through the middle ear acts on the basilar membrane producing a traveling wave that excites the sensory cells in the Organ of Corti causing an auditory sensation. However, for certain ear canal and middle ear disorders (e.g. congenital malformations, ossicular discontinuity, otosclerosis, a perforated tympanic membrane, or chronic ear infections) air conduction hearing aids cannot be used or are insufficient. In such cases, bone conduction hearing aid can be provided as an alternative.
In bone conduction (BC) hearing aids, airborne sound picked up by the hearing aid is converted into vibrations that are transmitted through the skin to the skull bone and then directly to the cochlea. This bypasses the outer and middle ears. For patients with a functioning cochlea, such devices can substantially restore normal hearing. Bone conduction hearing aids can also be useful for patients with complete unilateral hearing loss because the skull can efficiently transmit vibrations from the deaf side to the functioning cochlea on the other side of the head (known as transcranial transmission). The use of bone conduction hearing aids for patients with unilateral hearing loss can significantly reduce the head shadow effect where sounds occurring on a patient's deaf side are attenuated by their own head before reaching their good ear.
In conventional BC hearing aids, the vibrator unit is pressed against the skull through the use of a headband or by mounting the vibrator unit to spectacle frames. Such devices can be uncomfortable due to the static force required to adequately couple the vibrator unit to the skull and suffer from reduced high frequency sensitivity due to the attenuation effect of soft tissue overlying the skull, as well as feedback problems due to sound radiation from the vibrator unit being picked up by the microphone of the hearing aid. These problems led to the development of a new form of bone conduction hearing aid: percutaneous bone anchored hearing aids (BAHAs).
Bone anchored hearing aids are clipped onto a bone anchor which is mounted in the skull and penetrates out through the skin. This approach provides good coupling between the vibrator unit of the hearing aid and the skull but the skin penetration site needs lifelong daily care. Some patients may acquire a skin reaction with persistent infection and may form granulation tissue that requires surgical revision or re-implantation. The bone anchor may also suffer from problems and can be damaged through injury. Some patients will refuse a BAHA because they cannot tolerate a skin penetration implant for personal reasons or social stigma.
In response to the problems with bone anchored hearing aids, subcutaneous bone conduction implant (BCI) devices are now becoming commonplace. These devices comprise an implanted unit which is fixed into a cavity drilled out of the temporal bone of the skull. The skin is replaced over the implanted unit so there is no part of the device penetrating the skin. The implanted unit contains a vibrator which is inductively driven by an external unit held in place over the implanted unit by a strong magnet. The inductive link consists of a transmitter coil at the external unit and a receiver coil at the implanted unit: by appropriately driving the transmitter coil, a current can be induced in the receiver coil sufficient to power the vibrator. Typically, the inductive transmission carries an amplitude modulated signal which can be demodulated at the implanted unit so as to cause the vibrator to reproduce the sounds picked up at the external unit.
BCI devices bring their own set of problems. The area of skin overlying the implanted unit must be kept free of hair which for most patients means regular shaving and an unsightly bald patch. The external unit is prone to falling off when the wearer performs physical activity and can be easily knocked off. Even so, the strength of the magnet used to hold the external unit in place leads to compression of the skin, leading to irritation and potentially pressure sores.
The operation required to install the implanted unit behind the ear in the temporal bone carries a high risk of facial nerve or vestibular damage. A large volume of bone (typically around 16×16×8 mm) must be removed to create a cavity in which the implanted unit is fixed which is irreversible and can weaken the temporal bone which lies only a few millimeters over the brain. When the implanted unit needs replacing, a further surgery is required. The implanted unit must also be removed should the patient require an MRI scan, which necessitates another risky operation. Even if the patient is not harmed by the action of the strong magnetic field present in an MRI scanner on the implanted unit, the implanted unit is likely to be damaged (e.g. due to demagnetisation of the biasing magnets of the vibrator). The MRI images in the region of the implanted unit would in any case be heavily distorted due to the presence of biasing magnets in the vibrator.
A variation on the BCI device is also used which replaces the vibrator at the temporal bone with a tiny vibrating unit that is fixed to one or the bones of the middle ear (the ossicles). The implanted unit is connected to the vibrating unit by thin wires which run through a channel drilled through the temporal bone. The implanted unit is inductively powered by an external unit in the manner described above. Sound received at the external unit is thus recreated by the vibrating unit as vibrations in the ossicles of the middle ear. Such an arrangement is not suitable for use in patients with unilateral hearing loss due to problems in the middle or inner ear because the device does not generate vibrations in the temporal bone which can be conducted across the skull to the functional cochlea. The vibrations generated in the middle ear by the vibrating unit are small in magnitude and confined to the ossicles.
The surgical risks associated with the implantation of a BCI device provided with a vibrating unit for attachment into the middle ear is even greater than the risks associated with installing conventional BCI devices. In addition to creating a cavity in the temporal bone, a channel must be drilled through the temporal bone in order to (a) access the middle ear and (b) provide a route for the wires between the implanted unit and vibrating unit. Operating on the middle ear increases the risk of infection and is a difficult operation to perform due to the very small size of the ossicles. Furthermore, even with the implanted unit removed and the vibrating units adapted to present a low magnetic moment, it is risky for a patient to undergo an MRI scan because of the fragility of the ossicles and the soft structures of the middle and inner ear.
U.S. Pat. No. 6,643,378 describes an alternative type of bone conduction hearing aid which is located in the ear canal rather than behind the ear. The hearing aid comprises a piezoelectric vibrator instead of the conventional speaker unit. Vibrations generated by the vibrator are transmitted into the mastoid bone through the casing of the hearing aid so as to conduct sound into the cochlea. This design suffers from several problems, most notably poor performance due to the low coupling of vibrations into the mastoid bone due to absorption by the casing of the hearing aid and the soft tissues lining the ear canal. In order to achieve an acceptable level of performance, the hearing aid must form a tight fit in the ear canal. This leads to irritation of the lining of the ear canal and potentially pressure sores. Since the hearing aid blocks the ear canal, use of the device creates conditions in the ear which can lead to infection. Finally, and significantly to wearers of the device, the vibration of the device in the ear causes a tickling sensation in the ear of the wearer which for some patients means using the device for extended periods of time is not an option.